PEAKS AND MOUNTAIN RANGES.
The lunar features next in order of conspicuity are the mountain ranges,
peaks, and hill-chains, a class of eminences more in common with
terrestrial formations than the craters and circular structures that
have engaged our notice in the preceding chapters.
In turning our attention to these features, we are at the outset struck
with the paucity on the lunar surface of extensive mountain systems as
compared with its richness in respect of crateral formations; and a
field of speculation is opened by the recognition of the remarkable
contrast which the moon thus presents to the earth, where mountain
ranges are the rule, and craters like the lunar ones are decidedly
exceptional. Another conspicuous but inexplicable fact is that the most
important ranges upon the moon occur in the northern half of the visible
hemisphere, where the craters are fewest and the comparatively
featureless districts termed “seas” are found. The finest range is that
named after our Apennines and which is included in our illustrative
Plate, No. IX. It extends for about 450 miles and has been estimated to
contain upwards of 3000 peaks, one of which—Mount Huyghens—attains the
altitude of 18,000 feet. The Caucasus is another lunar range which
appears like a diverted northward extension of the Apennines, and,
although a far less imposing group than the last named, contains many
lofty peaks, one of which approaches the altitude assigned to Mount
Huyghens while several others range between 11,000 and 14,000 feet high.
Another considerable range is the Alps, situated between the Caucasus
and the crater Plato, and reproduced on Plate XIV. It contains some 700
peaked mountains and is remarkable for the immense valley, 80 miles long
and about five broad, that cuts it with seemingly artificial
straightness; and that, were it not for the flatness of its bottom,
might set one speculating upon the probability of some extraneous body
having rushed by the moon at an enormous velocity, gouging the surface
tangentially at this point and cutting a channel through the impeding
mass of mountains. There are other mountain ranges of less magnitude
than the foregoing; but those we have specified will suffice to
illustrate our suggestions concerning this class of features.
[Illustration: PLATE XV.
MERCATOR & CAMPANUS.]
We remark, too, that there is a prevailing tendency of the ranges just
mentioned to present their loftiest constituents in abrupt terminal
lines, facing nearly the same direction, the reverse of that towards
which they are carried by the moon’s rotation; and as they recede from
the high terminal line, the mountains gradually fall off in height, so
that in bulk the ranges present the “crag and tail” contour which
individual hills upon the earth so frequently exhibit.
Isolated peaks are found in small numbers upon the moon; there are a few
striking examples of them nevertheless, and these are chiefly situated
in the mountainous region just alluded to. Several are seen to the east
(right hand) of the Alpine range depicted on Plate XIV. The best known
of these is Pico, which rises abruptly from a generally smooth plain to
a height of 7000 feet. It may be recognized as the lower of the two long
shadowing spots located almost centrally above the crater Plato in the
illustration just mentioned. Above it, at an actual distance of 40
miles, there is another peak (unnamed) about 4000 feet high; and away to
the west, beyond the small crater joined by a hill-ridge to Plato, is a
third pyramidal mountain nearly as high as Pico.
It seems natural to regard the great mountain chains as agglomerations
of those peaks of which we have isolated examples in Pico and its
compeers, and thus to consider that the formation of a mountain chain
has been a multiplication of the process that formed the single
pyramid-shaped eminences. At first thought it might appear that the
great mountain ranges were produced by bodily upthrustings of the crust
of the moon by some subsurface convulsions. But such an explanation
could hardly hold in relation to the isolated peaks, for it is
difficult, if not impossible, to conceive that these abrupt mountains,
almost resembling a sugarloaf in steepness, could have been protruded en
masse through a smooth region of the crust. On the contrary it is quite
consistent with probability to suppose that they were built up by a slow
process somewhat analogous to that to which we have ascribed the piling
of the central cones of the great craters. We believe they may be
regarded as true mountains of exudation, produced by the comparatively
gentle oozing of lava from a small orifice and its solidification around
it; the vent however remaining open and the summit or discharging
orifice continually rising with the growth of the mountain, as indicated
in the annexed cut, Fig. 36. This process is well exemplified in the
case of a water fountain playing during a severe frost; the water as it
falls around the lips of the orifice freezes into a hillock of ice,
through the centre of which, however, a vent for the fluid is preserved.
As the water trickles over the mound it is piled higher and higher by
accumulating layers of ice, till at length a massive cone is formed
whose height will be determined by the force or “head” of the water.
Substitute lava for water and we have at once a formative process which
may very fairly be considered as that which has given rise to the
isolated mountains of the moon.
[Illustration: Fig. 36.]
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
There are upon the earth mountainous forms resembling the isolated peaks
of the moon, and which have been explained by a similar theory to the
above. We reproduce a figure of one observed by Dana at Hawaii (Fig.
37), and a sketch of another observed on the summit of the Volcano of
Bourbon, (Fig. 38); we also reproduce (Fig. 39) an ideal section of the
latter, given by Mr. Scrope, and showing the successive layers of lava
which would be disposed by just such an action as that manifested in the
case of the freezing fountain; and we quote that author’s words in
reference to this explanation of the formation of Etna and other
volcanic mountains. “On examining,” says Mr. Scrope,[11] “the structure
of the mountain (Etna) we find its entire mass, so far as it is exposed
to view by denudation or other causes (and one enormous cavity, the Val
de Bove penetrates deeply into its very heart), to be composed of beds
of lava-rock alternating more or less irregularly with layers of scoriæ,
lapillo and ashes, almost precisely identical in mineral character, as
well as in general disposition, with those erupted by the volcano at
known dates within the historical period. Hence we are fully justified
in believing the whole mountain to have been built up in the course of
ages in a similar manner by repeated intermittent eruptions. And the
argument applies by the rules of analogy to all other volcanic
mountains, though the history of their recent eruptions may not be so
well recorded, provided that their structure corresponds with, and can
be fairly explained by this mode of production. It is also further
applicable, under the same reservation, to all mountains composed
entirely, or for the most part, of volcanic rocks, even though they may
not have been in eruption within our time.”
To these illustrations furnished from Scrope’s work we add another,
copied from a photograph by Professor Piazzi Smyth, of a “blowing cone”
at the base of Teneriffe (Fig. 40), which is but one of many that are to
be found on that mountain and which has been formed by a process similar
to that we have been considering, but acting upon a comparatively small
scale. Professor Smyth describes this cone as about 70 feet high and of
parabolic figure, composed of hard lava and with an upper aperture still
yawning, “whence the burning breath of fires beneath once issued in fury
and with destruction.”
[Illustration: PLATE XVI
TYCHO,
AND ITS SURROUNDINGS.]
Reverting now to the moon, we remark that, if the foregoing explanation
of the isolated lunar peaks be tenable, it should hold equally for the
groups of them which we see in the lunar Apennines, Alps, Caucasus and
other ranges of like character. There occur in some places intermediate
groups which link the one to the other. Just above the crater
Archimedes, on Plate IX., for instance, we see several single peaks and
small clumps of them leading by successive multiple-peak examples to
what may be called chains of mountains like many that are included in
the contiguous Apennine system. And, in view of this connexion between
the single peaks and the mountain ranges formed of aggregations of such
peaks, it seems to us reasonable to conclude that the latter were formed
by the comparatively slow escape of lava through multitudinous openings
in a weak part of the moon’s crust, rather than to suppose that the
crust itself has been bodily upheaved and retained in its disturbed
position. The high peaks that many mountains in such a chain exhibit
accord better with the former than the latter explanation; for it is
difficult to imagine how such lofty eminences could be erected by an
upheaval, and we must remember that the moon has none of the denuding
elements which are at work upon the earth, weather-wearing its mountain
forms into sharpness and steepness.[12]
[Illustration: Fig. 40.
SMALL VOLCANIC MOUNTAIN AT THE END OF A STREET AT TENERIFFE.]
[Illustration: Fig. 41.]
And we have ground for believing the mountain-forming process on the
moon to have been a comparatively gentle one, in the fact that the
mountain systems appear in regions otherwise little disturbed, and where
craters, which have all the appearances of violent origin, are few and
far between. Evidently the mountain and crater-forming processes,
although both due to extrusive action, were in some measure different,
and it is reasonable to suppose that the difference was in degree of
intensity; so that while a violent ejection of volcanic material would
give rise to a crater, a more gradual discharge would pile up a
mountain. In this view craters are evidences of _eruptive_, and
mountains of comparatively gentle _exudative_ action.
We can hardly speculate with any degree of safety upon the cause of this
varying intensity of volcanic discharge. We may ascribe it to variation
of _depth_ of the initial disturbing force, or to suddenness of its
action; or it may be that different degrees of fluidity of the lava have
had modifying effects; or on the other hand different qualities of the
crust-material; or yet again differences of period—the quieter
extrusions having occurred at a time when the volcanic forces were dying
down. There is an alliance between lunar craters and mountains that goes
far to show that there has been no radical difference in their origins.
For instance, as we have previously pointed out, craters in some cases
run in linear groups, as if in those cases they had been formed along a
line of disruption or of least resistance of the crust; and the mountain
chains have a corresponding linear arrangement. Then we see craters and
mountain chains disposed in what seem obviously the same arcs of
disturbance. Thus Copernicus (No. 147), Erastothenes (No. 168), and the
Apennines appear to belong to one continuous line of eruption; and it
requires no great stretch of imagination to suppose that the Caucasus,
Eudoxus (No. 208) and Aristotle (No. 209) form a continuation of the
same line. Then around the Mare Serenetatis we see mountainous ridges
and craters alternating one with the other as though the exuding action
there, normally sufficient to produce the ridges, had at some points
become forcible enough to produce a crater. Again, upon the very
mountain ranges themselves, as for instance among the Apennines, we find
small craters occurring. We see, too, that the great craters are in many
cases surrounded by radiating systems of ridges which almost assume
mountainous proportions, and which are doubtless exuded matter from
“starred” cracks, the centres of which are occupied by the craters. The
same kind of ridges here and there occur apart from craters (see for
instance Plate XVIII., below Aristarchus and Herodotus) and sometimes
they occur in the neighbourhood of extensive cracks, to which they also
seem allied. We must indeed regard a linear crack as the origin either
of a ridge (if the exudation is slight) or of a mountain chain (if the
exudation is more copious) or a string of craters (if the extrusion
rises to eruptive violence). But the subject of cracks is important
enough to be treated in a separate chapter.
We alluded in Chap. III. to the phenomena of wrinkling or puckering as
productive of certain mountainous formations; and we pointed out the
striking similarity in character of configuration between a shrivelled
skin and a terrestrial mountain region. We do not perceive upon the moon
such a decided coincidence of appearances extending over any
considerable portion of her surface; but there are numerous limited
areas where we behold mountainous ridges which partake strongly of the
wrinkle character; and in some cases it is difficult to decide whether
the puckering agency or the exudative agency just discussed has produced
the ridges. The district bordering upon Aristarchus and Herodotus, above
referred to, is of this doubtful character; and a similar district is
that contiguous to Triesnecker (Plate XI.) There are, however, abundant
examples of less prominent lines of elevation, which may, with more
probability, be ascribed to a veritable wrinkling or puckering action;
they are found over nearly the whole lunar surface, some of them
standing out in considerable relief, and some merely showing gentle
lines of elevation, or giving the surface an undulating appearance. A
close examination of our picture-map (Plate IV.) will reveal very
numerous examples, especially in the south-east (right-hand-upper)
quadrant. Some of these lines of tumescence are so slightly prominent
that we may suppose them to have been caused by the action indicated by
Fig. 6 (p. 28), while others, from their greater boldness, appear to
indicate a formative action analogous to that represented by Fig. 9 (p.
29).
[Illustration: IDEAL SKETCH OF PICO AS IT WOULD PROBABLY APPEAR IF
SEEN BY A SPECTATOR LOCATED ON THE MOON.]
[Illustration: PLATE XVII.
WARGENTIN.]